In relatively small displacement multicylinder internal combustion engines of the type having overhead valve gear, for example, four-cylinder in-line engines, it is not uncommon for such engines to operate at high rotational speeds, for example, 4000-6000 r.p.m. during a large percentage of the normal duty cycle. In this high r.p.m. regime, the inertial forces of the valve gear can become critical with respect to the valve spring closing force. In order to reduce the inertial forces of the valve gear, it is desirable to eliminate the pushrod type valve gear and utilize a direct acting or cam-over-rocker arrangement. This arrangement eliminates the tappets and pushrods between the camshaft and the valve rocker arms. However, in designing valve gear for a cam-over-rocker arrangement where the cam lobes contact the rocker arms directly, the usual technique, employed in pushrod-type valve gear of providing lash adjustment in the tappet, is not available. Whereas, in the conventional pushrod-type valve gear, the lash adjustment is usually provided in the form of a combination tappet-hydraulic valve lifter between the pushrod and the camshaft. Where attempts have been made to utilize hydraulic lash adjusters for cam-over-rocker valve gear, it has been found that the most compact arrangement is to provide the lash adjustment at a stationary pivot about which one end of the valve rocker is pivoted. However, where the lash adjusters operate from the pressurized engine oil, the force obtained from the oil pressure acting on the end of the adjuster plunger is increased by the mechanical leverage of the rocker and applied to the cam lobe at the rocker contact surface. This additional force results in the need for greater valve spring forces at high r.p.m.
Furthermore it has been found that, upon starting of cold engines having the cam-over-rocker type valve gear with hydraulic lash adjusters supplied by the engine lubricant system, at high r.p.m. while the engine is cold, extremely high oil pressure conditions are experienced. For example, it has been determined that high r.p.m. operation of a cold engine has produced engine lubricant system pressures in the range of 85 to 130 p.s.i.; whereas, ordinary engine operating conditions produce only a maximum lubricant pressure of 65 p.s.i. at maximum r.p.m.
In engines of the type having cam-over-rocker valve gear and employing stationary hydraulic lash adjusters supplied from the engine lubricant system, this high oil pressure, in the range of 85 to 130 p.s.i. acting across the surface of the lash adjuster plunger, provides a sufficient axial force on the plunger to pivot the rocker arm about a fulcrum, located at the point of contact of the rocker arm with the cam lobe, with a force sufficient to overcome the closing force of the engine poppet valve spring. Thus, a situation occurs where, at high r.p.m. with a cold engine, the hydraulic lash adjusters hold the engine valves open, and this condition is sometimes referred to as pump-up. This condtion will generally persist until the engine reaches normal operating temperature where the oil pressure decreases to the normal operating range. This decrease in oil pressure decreases the force of the lash adjuster plunger on the rocker arm and permits the valve springs to properly close the valves during each cam event.